The present invention is generally directed to thermal ink jet printing. More particularly, the invention is directed to a method and apparatus for maintaining desired levels of heat energy transferred into ink to form ink droplets as characteristics of an ink jet print head change over its operational lifetime.
Generally, thermal ink jet print head chips consist of several thin film layers, including a resistor layer, conductor layer, dielectric layer, and protection layer. When electrical current is passed through a resistive heating element formed in the resistor layer, ink adjacent to the heating element is superheated and forms a bubble that causes an ink droplet to be expelled from an adjacent nozzle.
Many thermal ink jet print heads incorporate a tantalum aluminum (TaAl) thin film as the resistor layer in which the resistive heating elements are formed. Over time, a TaAl thin film experiences material degradation due to current and temperature stressing as electrical current pulses are applied to the heating elements. The material degradation mechanisms include aluminum segregation from the TaAl film, recrystallization of the TaAl under high temperatures, and electromigration of aluminum from the TaAl film. This degradation causes a gradual decrease in the electrical resistance of the heating elements over time.
Many current ink jet printers apply one voltage level (rail voltage) to the resistive heating elements to pass electrical current through the elements, and this voltage level is not changed over the lifetime of a print head. With a constant rail voltage, any decrease in heating element resistance, such as by material degradation, causes a corresponding increase in the current flowing through the heating elements. An increase in current causes a corresponding increase in the heat energy generated by the heating elements, and an increase in the temperature at the surface of the heating elements. If surface temperatures rise too high, extensive ink kogation may occur at the surface of the heating elements. Also, increased current levels cause even greater electromigration or segregation of the aluminum in the TaAl film, which is further detrimental to heater reliability.
Therefore, a system is needed for maintaining stable heat energy levels at the surfaces of the resistive heating elements over the operational lifetime of an ink jet print head.
The foregoing and other needs are met by a method of operating a thermal ink jet print head having nozzles through which ink is ejected when energy pulses having a desired pulse energy are applied to resistive heating elements associated with the nozzles. Each of the resistive heating elements has a heater resistance which tends to change over the operational lifetime of the print head. The method provides stable ink ejecting characteristics over the lifetime of the print head by compensating for the change in heater resistance. The method includes applying energy pulses having a first pulse width to the resistive heating elements, and counting the energy pulses to determine a pulse count. When the pulse count exceeds a threshold value, pulses having an adjusted pulse width are applied to the resistive heating elements, where the adjusted pulse width accounts for the changes in the heater resistance during the operational lifetime of the print head.
Preferred embodiments of the method include accessing a total print head resistance value which is based at least in part upon the heater resistance and resistances of circuit components in series with the resistive heating elements, accessing a heater resistance value related to the heater resistance, accessing a print head voltage value, accessing a first pulse energy value related to the desired pulse energy, and determining the first pulse width based upon the heater resistance value, the total print head resistance value, the print head voltage value, and the first pulse energy value. Preferred embodiments further include accessing a second pulse energy value related to the desired pulse energy and determining the adjusted pulse width based upon the heater resistance value, the total print head resistance value, the print head voltage value, and the second pulse energy value.
In another aspect, the invention provides a thermal ink jet printing apparatus for maintaining stable printing characteristics. The apparatus includes an ink jet print head having resistive heating elements for receiving electrical energy pulses having a voltage level and for transferring heat energy pulses having a desired energy level into adjacent ink based on the electrical energy pulses. The print head includes nozzles associated with the resistive heating elements through which droplets of the ink are ejected when the heat energy pulses are transferred into the ink. The apparatus further includes a printer controller in electrical communication with the print head. The printer controller determines a pulse count indicative of a number of electrical energy pulses, applies the electrical energy pulses having a first pulse width to the resistive heating elements when the pulse count is less than a threshold value, and applies the electrical energy pulses having an adjusted pulse width to the resistive heating elements when the pulse count exceeds the threshold value. The differences in the first and the adjusted pulse widths compensate for changes in the electrical resistance of the resistive heating elements over the operational lifetime of the print head.